Studies on oxygen toxicity will be conducted using a freshwater teleost retinal preparation. Criteria for O2 toxicity will be disappearance of ERG following exposure to hyperbaric pressures of oxygen. Contrary to the finding in other animals it is believed that the teleost retina, which is normally exposed to pressures of oxygen in excess of 400 mm Hg, will show little toxicity. The generation of superatomospheric O2 tensions by the fish eye has been shown to be dependent on carbonic anhydrase. The precise cellular location of this enzyme is critical to our further understanding of its role in the oxygen concentrating mechanism. Techniques of light and electron microscopy and radioautographs using 131I labeled carbonic anhydrase will be used. The dissipation of the O2 gradient throughout the ocular fluid will be determined in order to evaluate the choroidal rete as a source of oxygen for ocular metabolism. Evaluation of the choroidal rete as a counter current exchanger for other metabolic substrates will be attempted. Initial studies will deal with heat transfer and involve a determination of total heat production and heat loss i.e., thermal conductivity of ocular structures.